Acid mine drainage is a prevalent environmental issue throughout the world stemming from historic and active coal and hard rock mining activities. One issue, especially with historic sites, is how the waste material was handled and the location of the waste disposal sites. For instance, this waste material was usually just piled up on the surface near the mine site where it is susceptible to oxidative weathering and drainage into nearby streams. Much of this waste material may be composed of sulfidic ore as pyrite (FeS2) or other metal-sulfides that can oxidize and dissociate into sulfate and protons (acidity) when exposed to dissolved oxygen in water, thus, lowering the pH of the surrounding environment. This oxidation reaction and the dissolution of more minerals may be exacerbated by the presence of acidophilic Acidithiobacillus ferrooxidans, which may catalyze the oxidation of these metal sulfide minerals and result in even more acid mine drainage generation. As the most visual form of mine wastes, acid mine wastewater may contaminate watersheds with elevated concentrations of heavy metals and acidic waters, posing significant threats to the environment and human health. Acid mine wastewater may include, but is not limited to, acid mine drainage, mineral mine drainage, heavy metal containing effluent, and the like. Acid mine wastewater may be formed through an oxidation of metal sulfides, which may result in a release of protons (acidity) and hazardous metals.
Past methods used to treat exposed mine waste may include removal of the material and storing it in a secure location or perhaps even capping the material in place with impermeable or even semi-permeable barriers designed to eliminate airborne dust and reduce water infiltration into the pile and subsequent acid mine drainage generation. These caps may be composed of a variety of materials that range from cementitious capping materials to living blankets of acid/metal-tolerant grass, shrub, and tree species growing on thin layers of clay and top soil on top of the waste material. Other treatments for acid mine wastewater may include using minerals such as limestone to neutralize acidic water; utilizing natural processes (sulfate reducing activity) in ponds and wetlands; and perhaps treating acid mine wastewater using various bioreactors that dominantly utilize the activity of sulfate reducing bacteria. Yet, other past treatments may include use of microorganisms to reduce sulfate into sulfide resulting in metal-sulfide precipitates and proton consumption.
Reductive degradation, such as reductive dechlorination is a biological process that is used to describe certain types of biodegradation of oxidized metals, solvents, perhaps even chlorinated solvents, and the like in groundwater. Various chlorinated compounds such as but not limited to tetrachloroethylene, trichloroethylene, other chlorinated aliphatic hydrocarbons, aromatic hydrocarbons, and the like in groundwater can be biodegraded by naturally occurring bacteria. This may occur when other bacteria, perhaps even anaerobic bacteria, present in a contaminated site take electrons from organic compounds (the “electron donors”) and produce H2. Dechlorinating bacteria may use the electrons in the H2 to replace a chlorine atom in the compounds. If the site soil and groundwater contain organic electron donors, this process can proceed until all of the chlorine atoms, or other halogens, are removed. TCE may be dechlorinated via dichloroethene and vinyl chloride to ethylene gas, a harmless end-product. Other solvents, such as 1,1,1-TCA and carbon tetrachloride, can also be degraded by reductive dechlorination.